Re: [PATCH 03/13] scsi: unify allocation of scsi command and sensebuffer

[Boaz Harrosh]

On 05/26/2009 05:47 PM, James Bottomley wrote:
> 
> There are several other approaches:
> 
>      1. Keep the sense buffer packed in the command but disallow DMA to
>         it, which fixes all the alignment problems.  Then we supply a
>         set of rotating DMA buffers to drivers which need to do the DMA
>         (which isn't the majority).

This one is not possible because it is scsi-ml in majority of cases that
does the DMA request through scsi_eh_prep_cmnd() and a regular read.
The drivers don't even know anything about it.

>      2. Sense is a comparative rarity, so us a more compact pooling
>         scheme and discard sense for reuse as soon as we know it's not
>         used (as in at softirq time when there's no sense collected).
> 

This is the way to go for sure. And only on ARCHs with none-coherent-cache
all the good ARCHs can just use embedded sense just fine.

> I'd need a little more clarity on the actual size of the problem before
> making any choices.
> 
> The other thing to bear in mind is that two allocations of M and N might
> be more costly than a single allocation of N+M; however, an allocation
> of M+N+extra can end up more costly if the extra causes more page
> reclaim before we get an actual command.
> 
> James
> 
Boaz
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